The present invention relates to an aqueous emulsion having excellent polymerization stability and standing stability. More specifically, the present invention relates to an aqueous emulsion, in which polymer particles comprising an acrylic monomer which have polyvinyl alcohol containing an acetoacetic ester group with little difference in acetoacetic esterification degree by particle size and high blockiness attached to the surface thereof are the dispersoid, and a redispersible powder obtained by drying the aqueous emulsion. The aqueous emulsion and redispersible powder of the present invention can be used as an adhesive for particleboards and an additive for cement and mortar.
Conventionally, in order to give mechanical stability and frozen stability to an acrylic aqueous emulsion, polyvinyl alcohol (hereinafter referred to as PVA) is used as a protective colloid. However, although mechanical stability and frozen stability of the emulsion are improved, polymerization stability is insufficient and particularly, when the resin content in the emulsion is a high concentration of more than 50% by weight, polymerization cannot be conducted. Therefore, when PVA is used as a protective colloid of an acrylic aqueous emulsion, the resin content in the emulsion must be at most 50% by weight, which is a problem in terms of productivity. Also, the stability of the obtained emulsion is insufficient and there is the problem that viscosity increases over time.
Also, the vinyl acetate or acrylic aqueous emulsion and the redispersible powder obtained by drying the aqueous emulsion are used for mixing in cement and mortar. However, when the emulsion is mixed, there is the problem that flowability of the cement and mortar becomes poor over time and workability decreases.
Therefore, desired is development of a PVA protective colloid, by which polymerization can be conducted even when the resin content in the emulsion is a high concentration of at least 50% by weight, in the case that PVA is used as a protective colloid of an acrylic aqueous emulsion, and an aqueous emulsion or redipsersible powder, which does not decrease workability when mixed in cement and mortar.
As a means therefor, the method of using polyvinyl alcohol containing an acetoacetic ester group as an emulsifying dispersion agent is suggested and described in many patent documents. For example, JP-A-9-110925 discloses a process for preparing PVA containing an acetoacetic ester group (hereinafter referred to as AAPVA). AAPVA, in which the value obtained by dividing the maximum value with the minimum value of the acetoacetic esterification degree (hereinafter referred to as AA degree) of each AAPVA separated by particle size of 44 to 74, 74 to 105, 105 to 177, 177 to 297, 297 to 500 and 500 to 1680 μm is 1.0 to 3.0, is described as having excellent transparency and stability as an aqueous solution and being suitable as an emulsifying dispersion agent.
JP-A-2003-277419 discloses an aqueous emulsion in which PVA containing an active hydrogen group such as an acetoacetic ester group, a mercapto group and a diacetone acrylamide group, which has block character [η] of more than 0.6, hydrolysis degree of more than 95.0% by mol and low blockiness, is attached to a polymer. The mechanical stability, frozen stability and high temperature storage stability of the obtained aqueous emulsion and the mechanical stability of the redispersible powder obtained by drying the emulsion are described as being favorable. Furthermore, Examples are described, wherein polymerization is conducted so that the resin concentration in the emulsion becomes 50% by weight.
JP-A-2002-60406 discloses a powder obtained by attaching PVA containing an acetoacetic ester group or a mercapto group and having high blockiness of block character [η] of 0.3 to 0.6 to a polymer comprising an ethylenic unsaturated monomer and/or a diene monomer. It is described that the obtained powder is excellent in redipersibility and the properties of cement in which the powder is mixed are favorable. Furthermore, Examples are described, wherein polymerization is conducted so that the resin concentration in the emulsion becomes 50% by weight.
However, in recent years, from the viewpoint of increased productivity, the emulsion is preferably polymerized with as high resin concentration as possible. In the aqueous emulsions of the above patent documents, when polymerization is conducted with resin concentration in the emulsion of at least 50% by weight, problems occur, such as coarse particles develop due to insufficient polymerization stability and the viscosity of the emulsion increases due to insufficient standing stability.
Also, in the case that the aqueous emulsions and the redispersible powder of the above patent documents are mixed in cement and mortar, workability is favorable when the cement and mortar are used immediately after mixing, but when used some time after mixing, flowability of the cement and mortar decreases and there is the problem that workability decreases.